During the past eight years at the Institute for Muscle Disease in New York City I carried out extensive studies of the Mg2 ion-ATPase and the F-actin-activated ATPase activities of skeletal muscle myosin. With this experience as a basis I propose to study the kinetics and the binding activities of ATP and adenylyl imidodiphosphate to myosins and actomyosins prepared from heart and from platelets. I shall also determine if heart and platelet myosins react with radioactive ATP and radioactive adenylyl imidodiphosphate to yield an (NH4)2SO4-precipitable complex. The properties of such complexes will be determined and compared to the complexes prepared from rabbit sketetal muscle myosin. The F-actin binding sites on these myosins will be determined. The F-actin-activated ATPase activities will be analyzed for their steady state kinetics. These myosins will be reacted with the affinity analogs of ATP (6-deoxy-6-thioinosinylyl imidodiphosphate and 8-azido ATP). The effect of covalent linkage of these analogs to the myosins on the ATPase activities will be determined.